FREDERICKSBURG, Va. – Federal investigators, unhappy at the length of time it took them to crack the sniper case, say the cornerstone of their investigation, the tip line which received 138,000 calls, hurt as much as it helped.

What threw investigators most, said FBI special agent Larry Foust, was the huge number of tips about white vans – which led to further tips about white vans to the exclusion of almost all other information.

Still authorities had no choice but to go with that lead, Foust said.

“If it’s not released and someone is killed by a guy in a white van, they’re going to come back to us and ask why that information wasn’t released,” he said.

Meanwhile, tipsters who phoned in accurate tips are wondering why they were ignored.

Laurel, Md., resident, Katrina Carroll, told The Post her call about an African-American in a blue Caprice was met with condescension.

“I felt like I needed to apologize for calling,” she said.

“Malvo or Muhammad said they were frustrated with the tip line. I don’t want to say I sympathize with them, but I understand their frustration.”